The primary function of a photoinitiator is to generate polymerization initiating radicals when the photoinitiator is irradiated with ultraviolet (UV) light. Photoinitiators are classified into “Type I” (or photocleavage) photoinitiators and “Type II” (or H-abstraction) photoinitiators according to the pathways by which the effective initiating radicals are generated.
In contrast to photocleavage photoinitiators which are decomposed by UV light directly into radicals which are effective in initiating polymerization, H-abstraction photoinitiators require a hydrogen donor, or more generally a source of abstractable hydrogens in order to generate radicals that are effective in initiating polymerization. The process of H-abstraction is usually a bimolecular reaction requiring the encounter of a photoinitiator and a hydrogen-donor. Any source of abstractable hydrogens may be useful (e.g., any structure that yields a stable radical after H-abstraction may serve as a “H donor”) and such sources include amines, thiols, unsaturated rubbers such as polybutadiene or polyisoprene, and alcohols.
The basic photochemistry and photophysics of both a-cleavage (Type I) and H-abstraction (Type II) photoinitiators has been well studied and utilized industrially in UV curable systems (see (a) Cowan, D. O.; Drisko, R. L. Elements of Organic Photochemistry, 1976, Plenum Press, chapters 3 & 4. (b) Turro, N. J. Modern Molecular Photochemistry, 1991, University Science Books, chapters 7, 10, & 13.). One well recognized problem with the use of UV curable systems for coating and adhesive applications is the fate of the photo-by products created by the curing process. In the case of typical α-cleavage type photoinitiators, the production of benzaldehyde (and often related compounds) is often a significant concern from both a toxicity and product odor standpoint. Such concerns become especially important when the use of radiation curable materials is considered for applications that involve skin or food contact. Various effective approaches have been taken to reduce the odor and extractable by-product content of UV curable materials. One approach has been the use of copolymerizeable or polymeric photoinitiators which are chemically incorporated into the cured polymeric matrix as opposed to remaining in the irradiated material as a small molecule (see (a) Fouassier, J. P.; Rabek, J. F., Eds. Radiation Curing in Science and Technology, 1993, Elsevier Appl. Sci., vol. 2, 283–321. (b) Fouassier, J. P. Photoinitiation, Photopolymerization and Photocuring, Fundamentals and Applications, 1995, Hanser Publishers, 71–73.). Unfortunately, when utilizing α-cleavage photoinitiators at least one of the cleavage by-products still remains as a small molecule even if the other fragment is incorporated into a polymeric component of the system. Thus, although extractable and odorous by-products can be reduced through the use of polymeric or polymerizeable Type I photoinitiators, they are not eliminated entirely.
The use of polymerizeable or polymeric H-abstraction type photoinitiators, in principle, presents the possibility of creating a system with zero extractable components related to the photoinitiation system. Various groups have presented systems based upon poly(vinyl benzophenone) and its copolymers or polymers derived from acrylated benzophenone derivatives (see (a) David, C.; Demarteu, W.; Geuskens, G. Polymer, 1969, 10, 21–27. (b) Carlini, C.; Ciardelli, F.; Donati, D.; Gurzoni, F. Polymer, 1983, 24, 599–606.). The direct use of acrylated benzophenones has also been disclosed (U.S. Pat. No. 3,429,852). Unfortunately, these Type II systems often suffer from issues related to photoefficiency relative to analogous small molecule photoinitation systems.
It is notable that, in products requiring very low extractable levels or odor, any copolymerizeable photoinitiator that does not fully react into the growing polymer network will remain in the final product as a small molecule, creating many of the same problems that photoinitiator fragments introduce. Thus, it is often most desirable to use polymeric photoinitiators as opposed to polymerizeable photoinitiators. A key practical issue when utilizing polymeric photoinitiators is their compatibility with the resins systems in which they are to be used. An additional requirement of such polymeric photoinitiators, particularly those intended for use in hot melt adhesives and coatings, is that that they be thermally stable at the application temperature. The polymeric Type II photoinitiators known in the prior art fail to meet one or more of the requirements and needs noted above.
There continues to be a need in the art for improved H-abstraction photoinitiators useful in the manufacture of radiation curable adhesives and coating formulations. In particular, polymer-bound H-abstraction photoinitiators are needed to produce low odor products with fewer (or no) inherent extractable photochemical by-products. The current invention fulfils this need.